Optical waveguides have on the one hand the form of fiber waveguides. On the other hand they are the basic elements of integrated optics, where they are either planar waveguides or strip waveguides, that are arranged on a substrate or immediately under its surface. The waveguides have a higher refractive index than that of the substrate or the superstrate by which they are covered. Therefore the guidance of optical waves in the waveguide by total internal reflection is possible. Further integrated optical elements are, in particular, y-junctions, beam splitters and lenses for guided waves, moreover gratings acting as input or output couplers or as Bragg reflectors. In particular surface relief gratings are used which are located at the interface between substrate and waveguide or at the waveguide surface, or if the latter is covered with a superstrate, at the interface between waveguide and superstrate. With such integrated optical elements, integrated optic circuits are composed.
According to the prior state of the art in particular the following integrated optic circuits are known, which are used in the present invention: directional couplers, which consist of two strip waveguides that in a several millimeter long coupling region have a very small distance from each other and which, for example, are used for switching optical waves between two different output ports and as intensity modulators; X-switches, where two strip waveguides cross each other under a small intersection angle, and which are used for switching optical waves between the two output waveguides; two-beam interferometers, in particular Mach-Zehnder and Michelson interferometers, which, e.g., are used for intensity modulation of optical waves; resonators, in particular Fabry-Perot, ring, and DFB (i.e., distributed feedback) resonators, which, e.g., are used as frequency filters and laser resonators; Bragg reflectors, which, e.g., are used as frequency selective reflectors or deflectors and for mode conversion between modes of different polarization and/or mode number; input grating couplers, which are used for incoupling of an optical wave into a waveguide; output grating couplers, which are used for outcoupling of a guided wave out of a waveguide, whereby the outcoupled beam can be focused by an output coupler with curved grating lines and non-constant grating period.
With such integrated optic circuits active integrated optic devices which can perform time dependent operations, in particular phase and intensity modulation, switch on and switch off, switching between different output ports, or deflection of guided optical waves, tuning of resonance frequencies of frequency filters or of the frequency for which the Bragg reflection law at a grating is satisfied, and tuning of the resonance frequencies of resonators, are realized according to the prior state of the art by exploiting the electro-optic, or more seldomly, the photo-elastic or the magneto-optic effect in the waveguide material itself or in the adjacent regions of the substrate or superstrate. The linear electro-optic effect occurs only in crystals but not in isotropic materials. This law of physics leads to a disadvantage in the prior art in that the choice of materials for active integrated optic devices is very restricted. Presently mainly LiNbO.sub.3 crystals and III-V semiconductor compounds are used. Since for fiber optic communication technology cheap intgrated optic circuits are required in large numbers, the use of other and cheaper materials would be very desirable.